public FullName  : string
    {
      get
      {
        match (this)
        {
          | ListItem as item => item.FullName
          | _ => this.Owner?.FullName
        }
      }
    }

    | ListItem                   { ParsingSequence : ParsingSequence; IsItem : bool; }

SP>    public FullName  : string
SP>    {
SP>      get
SP>      {
SP>        match (this)
SP>        {
SP>          | ListItem as item => item.FullName
SP>          | _ => this.Owner?.FullName
SP>        }
SP>      }
SP>    }
SP>

public string Fullname
{
  get 
  {
     if (this is ListItem)
       return ((ListItem) this).FullName;
     ...
  }
}